You’re referring to results of a long-running National Institutes of Health study of the effect of diet on the course of breast cancer. The 48,835 women participating were randomly assigned to lower their fat intake to 20 percent of daily calories or to eat as usual for 8.5 years. (The amount of fat in their usual diets averaged 32 percent of calories.) None of the women had breast cancer when they joined the study. All were between the ages of 50 and 79.
Over the years, 1,764 of them were diagnosed with breast cancer, but over time, those on the low-fat diet were 35 percent less likely to die of any cause than the women who made no dietary change. Even 20 years later, women who followed the low-fat diet had a 15 percent lower overall mortality rate and a 21 percent lower risk of dying from breast cancer than those in the other group.
I should mention that the women on the low-fat diet didn’t always manage to reduce their fat consumption to 20 percent of daily calories. Study leader Rowan Chlebowski, M.D., Ph.D., of the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center said that most of them got their fat consumption down to about 24.5 percent of daily calories, although this “drifted up to about 29 percent.” Even so, the women in this group who developed breast cancer had a lower risk of death than those who developed the disease and hadn’t changed their eating habits.
The low-fat diet the women were asked to follow was similar to the DASH diet (Dietary Approaches To Stop Hypertension) developed by the National Heart, Lung and Blood Institute, a branch of the National Institutes of Health. It involves consuming less salt and fat and eating more fruits, vegetables, whole grains, and low-fat dairy products than most people eat. It is low in saturated fat, total fat, and cholesterol and includes much less red meat and fewer sweets and sugared beverages than the typical American diet. The DASH diet is associated with lower risks of breast, colorectal and prostate cancer.
Other studies have shown that women with a higher intake of olive oil have less breast cancer. Omega-3 fats, found in cold-water fish (like salmon and sardines), freshly ground flaxseed, hemp seeds, and walnuts, also appear to inhibit the growth of breast tumors. I recommend taking two grams of supplemental fish oil daily as well as eating 8 to 10 servings a day of vegetables and fruit. And be aware that women who eat the most red meat have a significantly higher breast cancer risk than those who eat the least or none.
Andrew Weil, M.D.
Rowan T. Chlebowski et al, “Association of Low-Fat Dietary Pattern With Breast Cancer Overall Survival” presentation at American Society for Clinical Oncology Conference May 31 to June 4, 2019, Chicago, IL.